94 



TORTOISES AND TURTLES. 



of the presence of a pair of large shields on the outer side of the hind-foot of the 

 typical species — bring us to the second family of the group under consideration. 

 This family (Pelomedusidce), which contains three genera, and is now confined to 

 Africa, Madagascar, and South America, is broadly distinguished from the last by 

 having eleven elements in the plastron, owing to the presence of a pair of meso- 

 plastral bones ; while the neck is completely retractile within the margins of the 

 shell. The skull differs from that of the preceding family in having a bony 

 temporal arch, as shown in the figure on p. 89 ; while it lacks the distinct nasal 

 bones generally found in the former. 



The largest and best known representative of the whole family is the giant 

 Amazonian tortoise (Podocnemis expansa), which considerably exceeds in size all 



GIANT AMAZONIAN TORTOISE (\ liat. size). 



other members of the entire group, having a shell which may measure as much as 

 30 inches in length. It belongs to a genus including seven existing species, of 

 which six are South American, while the seventh is an inhabitant of Madagascar. 

 This extremely anomalous distribution is to some extent explained by the 

 occurrence of a fossil representative of the genus in the Eocene strata of India, 

 which probably indicates that these tortoises were at one time widely spread. As 

 a genus, these tortoises are characterised by the skull having a roof over its 

 temporal region, coupled with the presence of five claws on the fore-feet, and four 

 on the hinder pair, and likewise by the circumstance that the mesoplastral bones 

 are small and confined to the edges of the plastron, so that they are widely 



